Key systems and private branch exchanges (PBXs) are widely used telephone systems employed, primarily, with small and medium size businesses. PBX systems used in such businesses include both analog and digital PBX systems connected to two-wire analog trunks to connect their PBX or key system to the local phone company. Telephone lines which are used for residential purposes also typically are two-wire analog lines. Larger businesses, requiring many trunks, usually use systems known as T-1 digital trunks, because T-1 trunks provide the equivalent number of voice channels as twenty-four individual analog trunks.
The widespread use of personal computers (PCs) has resulted in the interconnection of such computers through modems which convert the digital signals from the computer to analog signals suitable for transmission over analog telephone lines, and for reconverting received analog signals into digital signals for processing by the computer. Early modems operated at relatively low speeds (below 28.8 kbps); and when such modems were connected between a personal computer and a digital or analog PBX, the modems operated at capacity equally as well with either type of PBX.
The introduction of high speed modems, currently operating at 28.8 kbps and 33.6 kbps, revealed a problem in conjunction with both digital key systems and digital PBXs. The problem manifested in unexpected operation of the modems below their capability. For example, when a user placed a high speed modem call through a digital PBX connected with a local two-wire analog telephone line, the operating speed of the modem dropped significantly. With a 28.8 or a 33.6 kbps modem, slower speeds (as low as 21.6 kbps or 19.2 kbps, or even lower) resulted. When the same modem was used in conjunction with a modem call directly without passing through a digital PBX, the full operating speed of the modem was realized. The slowdown only occurred with a system involving a digital PBX used in the loop.
Much higher speed modems, currently 56 kbps are in use; and such modems are not identified with the problem noted above. The reason is that 56 kbps modems require digital trunks, such as the T-1 trunk mentioned above or ISDN between the phone company and the digital PBX in order to achieve speeds greater than 33.6 kbps. The problem only arises when the digital PBX is coupled with a two-wire analog telephone line of the type commonly used throughout the United States and other countries.
From the foregoing, it was apparent that the digital PBX somehow adversely affected the modem speed under some operating conditions. The cause of the problem, however, was not at all apparent, and often has been accepted as some idiosyncracy that sometimes (but not always) affects the modem operation with systems employing digital PBXs.
After considerable analysis, it has been found that a combination of multiple conditions are required to cause the modem slowdown mentioned above. Even though a combination of all of these conditions was found necessary to create the problem, the combination occurred frequently enough that the problem has been relatively widespread. The conditions which were found to create the problem include, first of all, the connection of a high speed modem behind a digital PBX (not an analog PBX). The digital PBX then is connected to a local public switched telephone network (PSTN) using two-wire analog trunks. There is no problem when T-1 digital trunks are employed. Next, the two-wire analog trunk interface with the PBX uses industry standard companding non-linear codecs (e.g. xcexc-law or A-law) for making the necessary analog-to-digital conversions. In addition, the trunk interface for the PBX also uses industry standard analog hybrid circuits to perform the two-wire to four-wire audio conversion (and back again).
Another factor for creating the problem noted above is that the two-wire analog trunk to the local PSTN was unusually long, resulting in greater than normal loss to the audio level (typically, 8 to 9 dB of loss). Finally, for the slowdown of the modem to take place, the user needed to make a high speed modem call through the digital PBX via the unusually long two-wire analog trunk going to the PSTN with all of the other conditions mentioned above being present. With so many requirements needed to work together to cause the problem, it would seem that the problem would occur infrequently. Unfortunately, for many digital key systems and PBX users, this has not been the case; and the problem mentioned here is in fact quite common and very annoying. Two-wire analog trunks are still common, especially among smaller businesses that do not require the bandwidth of a digital T-1 trunk. In addition, a fair percentage of the two-wire analog trunks are longer than what is considered xe2x80x9cnormalxe2x80x9d, causing significant audio loss in the line. When this is coupled together with the specific system components involving the digital PBX mentioned above, the modem slowdown occurs.
It is desirable to provide a system using a digital PBX in conjunction with a two-wire analog telephone trunk, which permits full utilization of high speed modems in conjunction with such a system.
It is an object of this invention to provide an improved digital PBX operating system.
It is another object of this invention to improve the operating characteristics of a digital PBX used in conjunction with a high speed modem.
It is an additional object of this invention to provide an improved system for a digital PBX interfacing with a two-wire analog telephone trunk.
It is yet another object of this invention to provide an improvement in signal-to-noise (S/N) and trans-hybrid loss (THL) which results in improved audio allowing for more 2-wire analog trunk parties in a conference before xe2x80x9csingingxe2x80x9d or xe2x80x9csquealingxe2x80x9d occurs.
It is a further object of this invention to include a linear codec and a DSP-based echo cancellation circuit in conjunction with a digital PBX coupled with a two-wire analog telephone trunk for enhancing the operating performance of high speed modems used with the PBX.
In accordance with a preferred embodiment of this invention, a system is provided for ensuring optimum performance of a digital modem used in conjunction with a telephone system employing a digital PBX coupled with a two-wire analog trunk. This is accomplished by providing a linear codec and a DSP-based echo cancellation adaptive filter connected in series circuit between the digital PBX and the hybrid circuit which converts two-wire analog signals on the telephone trunk to four-wire signals for the PBX, and vice versa.